The usage of electro-photographic image forming apparatuses has expanded from office uses to printing fields. The electro-photographic image forming apparatuses have been widely used for forming full-color images. Due to the expansion of usage, image forming performance, which can exhibit high gloss and high brightness on various kinds of recording sheets, is required to the electro-photographic image forming apparatuses.
In order to meet the above requirement, a technology is offered as a fixing method for increasing the brightness (being smoothness) of the toner image carried on the recording sheet. For example, a recording sheet, carrying an un-fixed toner image, is processed by two fixing devices, that is, double fixing processes are carried out. However, if a recording sheet is processed by a fixing device having a high brightness performance, uneven brightness is adversely generated on a specific recording sheet by a curl flattening section and sheet ejection rollers, mounted at downstream of the fixing device. Accordingly, image forming apparatuses has been offered, carrying an uneven brightness preventing means, as shown in a following document.
An image forming system, described in Unexamined Japanese Patent Application Publication No. 2007-62970, is configured to include an intermediate sheet rejection unit, which can cool a recording sheet processed by a fixing means which is mounted between the fixing means within an apparatus and a sheet ejection option connected to the apparatus. Accordingly, the time interval, during which the specific recording sheet passes through the intermediate sheet ejection unit, is controlled to be longer than time interval during which other recording sheets pass though said unit. As result, the toner image on the specific recording sheet is cooled to a temperature at which no uneven brightness is generated by rollers mounted in said sheet ejection unit. Subsequently, said specific recording sheet is conveyed to said sheet ejection unit, so that no uneven brightness is generated by the above-described image forming system.
However, according to the above-described technology, the necessary time interval for processing a single specific recording sheet becomes longer than the time interval for processing a recording sheet other than the specific recording sheet, that is, an additional time interval, in which the recording sheet stops at the intermediate sheet ejection unit, is required for the single specific recording sheet. Accordingly, an image forming productivity for the specific recording sheet is adversely reduced to be less than the image forming productivity of the recording sheet other than the specific recording sheet.
Due to the above problem, the technology described in the above patent document is not suitable for the image forming system to be used in the office or the printing field, which requires a high-speed process.
An object of the present invention is to offer an image forming system which makes it possible to rapidly form high quality images exhibiting no uneven brightness, regardless of the type of the recording sheets.